1. FIELD OF THE INVENTION
The present invention relates to a pressure transducer, and more particularly to a pressure transducer having an integrated pressure sensor formed in a bridge circuit, and a temperature compensation circuit.
2. DESCRIPTION OF THE PRIOR ART
A semiconductor pressure transducer is known, in which a thin diaphragm is formed at a center of a silicon single crystal plate and gauging resistors are formed on the surface of the diaphragm by impurity diffusion layers, and the gauging resistors are connected to form a sensor of bridge circuit. Recently, a semiconductor pressure transducer has been proposed in which a temperature compensation circuit for a sensitivity of the sensor or an amplifier is integrated on the diaphragm surface or a peripheral thick area of the diaphragm. The sensitivity of the semiconductor pressure sensor is lowered as a temperature rises. Therefore, the temperature compensation is essential. The temperature compensation circuit may utilize a circuit called an nV.sub.BE network disclosed, for example, in U.S. Pat. No. 3,836,796 issued Sept. 17, 1974 to I. E. Solomon et al. and entitled "Semiconductor Pressure Transducer Employing Novel Temperature Compensation Means". The nV.sub.BE network utilizes a property that a base-emitter voltage V.sub.BE of a transistor decreases as a temperature rises. The nV.sub.BE circuit is connected in series with the bridge circuit of the pressure sensor and the series circuit is energized by a regulated power supply so that a voltage applied to the bridge circuit is raised by the amount corresponding to the reduction of V.sub.BE of the nV.sub.BE network due to the temperature rise in order to compensate for the reduction of the sensitivity. However, in the temperature compensation circuit which employs the nV.sub.BE network, the V.sub.BE voltage of the transistor is determined by a collector current and not influenced by the change of the power supply voltage. As a result, even if the power supply voltage varies, the amount of temperature compensation is substantially constant. On the other hand, the sensitivity of the bridge circuit changes in proportion to the power supply voltage. Accordingly, when the power supply voltage rises, the circuit is under-compensated and when the power supply voltage drops, the circuit is overcompensated. In the above U.S. Patent, a voltage regulator having a temperature compensation circuit by another nV.sub.BE network is used to overcome the above problem.
A temperature compensation circuit used to drive the bridge circuit of the pressure sensor with a constant current is disclosed in copending U.S. patent application Ser. No. 522,227 filed on Aug. 11, 1983 by K. Yamada et al. and entitled "Pressure Transducer with Temperature Compensation Circuit". The disclosed temperature compensation circuit basically resolves the above problem for the constant-current driven bridge circuit of the pressure sensor. A power amplifier of the bridge circuit may use the disclosed temperature compensation circuit to drive the bridge circuit with a constant voltage but it is basically not suitable to the constant-voltage driven bridge circuit.